1. Field of The Invention
The present invention relates to the field of biologic pump motor mechanisms. More specifically, it relates to an improved method for assisting the human heart in pumping blood to the body.
2. Description of the Prior Art
Heart failure is one of the leading causes of death in the United States, and is frequently caused by heart attacks. Heart attacks are most commonly caused when arteries supplying blood to a portion of the heart become blocked, resulting in that portion of the heart muscle dying off. The heart's left ventricle is its main pumping chamber. If a heart attack of any size occurs there, it will likely be fatal. If the area of heart muscle involved is not too large, the person will survive and that portion of the heart muscle will eventually become non-functioning scar tissue.
There are currently an estimated 10,000 patients in the United States annually suffering with irreversible congestive heart failure secondary to myocardial fibrosis. In such cases, the damaged heart may pump only a fraction of the amount of blood a normal one would. Fluids back up into the lungs, the lungs become engorged, and shortness of breath occurs. The ankles as well as the abdomen may become swollen. Physical activity is limited. Patients may only be able to take a few steps before becoming short of breath. Some are confined to bed.
Unfortunately the present outlook for most of these patients is dismal. Although conventional medical therapy with digitalis and after load reducing agents results in a mortality rate of approximately 50% within one year, it is the only method of treatment available for most patients. New drugs are always being developed and tested, but it is unlikely that any will be able to cause scar tissue that was once heart muscle to contract again.
Each year, more people benefit from heart transplants; however, heart transplantation continues to be plagued by numerous problems. About half of the patients who receive heart transplants die from rejection-related problems within five years after their transplant. The drugs used to suppress rejection are associated with numerous undesirable side effects and complications. There continues to be a significant donor shortage that frequently results in long waiting periods for these desperately ill patients. 10-25% of these patients may die while on the waiting list. In addition, because of the donor shortage, patients over the age of 65 are generally not even considered for such a surgical procedure. Cost is also a significant factor.
Mechanical hearts have been tried in a relatively small group of patients, but, at the present time, are not being used as a permanent heart replacement because of complications such as infection, stroke, and mechanical pump failure. They continue to be used as a "bridge to heart transplantation" for some of the patients with end-stage heart failure, a condition that will no longer allow them to wait for a suitable donor. This sometimes can buy a week or more of time while an appropriate donor heart is being located The use of these mechanical pumps in even these cases can be controversial since other short term support techniques are available. At rest, the patients who presently use such devices are tethered by tubes and wires to a large, cumbersome, external power source. Despite the fact that large sums of money are spent on this research each year by the United States government, there is no totally implantable power source even on the horizon.
Thus, there exists a need for a method or apparatus to aid individuals suffering from irreversible congestive heart failure.
In addition, in the United States, 12,000-15,000 babies are born each year with congenital heart defects. While the majority of these defects are relatively easy to correct surgically, the ideal form of correction for some congenital heart defects, such as hypoplastic left heart syndrome, has eluded the surgeon. Thus, there exists a need for a method or apparatus to aid in correcting the problems encountered by individuals suffering from such problems.
Over the years, there have been various suggestions to use autogenous skeletal muscle to either replace or assist the failing heart. Such methods have several advantages over other forms of therapy. First of all, the skeletal muscle is not foreign to the host and, therefore, would not be subject to tissue rejection. Secondly, there would be no problem of donor shortages. Thirdly, they would obviate the necessity for a cumbersome and inefficient external power source.
In 1933, Leriche and Fontaine experimentally demonstrated the feasability of using pectoralis muscle to reinforce myocardial scar after acute ligation of the coronary artery. Leriche, R., et al. "Essai experimental de traitment de certains infarctus du myocarde et de l'aneuvrisme de coeur par une greffe de muscle strie," 59 BULL. SOC. NAT. CHIR. 229 (1933). In 1935, Beck grafted skeletal muscle to the canine heart and then gradually occluded both coronary arteries, thereafter showing that the heart was given a new source of blood. Beck, C. S., "A New Blood Supply to the Heart by Operation," 61 S.G.O. 407 (1935). Kantrowitz was the first to wrap the aorta with skeletal muscle and then attempt to stimulate it in synchrony with the heart during cardiac diastole. He used the diaphragm muscle, and although he was able to show some increase in diastolic pressure, the effects lasted for less than a minute and then the muscle fatigued. See Kantrowitz and McKinnon, "The Experimental Use of the Diaphragm as an Auxiliary Myocardium", 9 SURG. FORUM 266 (1959). Others in the field performed variations on this theme but were limited by muscle fatigue.
In 1961, Petrovsky covered left ventricular aneurysms with diaphragmatic muscle flaps in an attempt to prevent further aneurysm enlargement and to induce collateral circulation with adjacent ischemic myocardial tissue Petrovsky, G. V., "Surgical Treatment of Cardiac Aneurysms," 41, J. THORAC. & CARDIOVASC SURG. 438 (1961). Drinkwater demonstrated in acute studies that portions of the left ventricle could be replaced with skeletal muscle. When a skeletal muscle was stimulated during cardiopulmonary bypass, the left ventricle was able to generate additional pressure. Dewar, M. L., et al, "Synchronously Stimulated Skeletal Muscle Graft for Myocardial Repair," 87 J. THORAC. & CARDIOVASC, SURG. 325 (1984).
In all of this early work, the primary impediment to the use of skeletal muscle to replace damaged myocardium or to function as a cardiac assist device was skeletal muscle fatigue. In recent years, however, various researchers have discovered that the problem of fatigue can be greatly reduced through the use of electrical conditioning of skeletal muscle. See, e.g., Macoviak et al, "Electrical Conditioning of In Situ Skeletal Muscle for Replacement of Myocardium", 32 J. SURGICAL RES. 429 (1982), incorporated herein by reference.
Cardiac muscle is similar to skeletal muscle with respect to the basic structure and mechanics of contraction, and in that both are capable of transforming chemically stored energy into mechanical work. The cardiac cell differs from the skeletal muscle cell by the presence of the intercalated disc. The intercalated disc is thought to serve as a low resistance pathway to facilitate electrical current flow during excitation, allowing the myocardium to contract in on "all or none" fashion. Unlike the heart, which is an electrical and mechanical syncytium, skeletal muscle is modulated by the number and rate at which the fibers are activated A single electrical stimulus resulting in a single muscle twitch does not generate sufficient force to power a cardiac assist device. However, rapid repetitive stimuli delivered before the muscle fiber completes its relaxation results in mechanical summation until fusion occurs, which thereby causes the muscle to generate substantially greater contractive force.
In order to optimally perform different functions, skeletal muscle has differentiated into two basic types of fibers. The Type I fiber is slow-twitch, oxidative, and found in muscles that are capable of generating substantial force over a prolonged period of time. They possess a prolonged contraction time, a large mitochondrial volume, a relatively small sarcoplasmic reticulum, and a specific slow type of myosin. Most importantly, these fibers resist fatigue and rely on an aerobic metabolism. The Type II fiber is fast-twitch, glycolytic, and more frequently found in muscles that generate intense, episodic movement, such as muscles controlling eye movement. These fibers possess a brisk contraction time, small mitochondrial volume, extensive sarcoplasmic reticulum, and a specific fast myosin molecule. The fast-twitch fibers are dependent on an anaerobic metabolism and fatigue rapidly.
It has been found that skeletal muscle is capable of changing its physiologic, biochemical, and structural characteristics in response to exercise and electrical stimulation. When a muscle is electrically stimulated by its motor nerve, the entire muscle undergoes transformation to the Type I muscle fiber. This process is complete after about six weeks of continuous stimulation with frequency as low as 2 Hertz (the frequency of a resting dog's heart rate). With this in mind, it has been found that it is possible to condition skeletal muscle while it is contracting in synchrony with the heart.
A variety of skeletal muscles have been investigated for use in cardiac assistance. A group including the inventors herein have decided that the latissimus dorsi is the most appropriate. This muscle was chosen for several reasons. It is a large, powerful muscle with about the same muscle mass as the left ventricle. It is a non-critical muscle; in fact, plastic surgeons use it with impunity to replace defects in the human abdominal or chest wall caused by tumors. They also occasionally use it to enlarge women's breasts. The loss of use of the muscle causes almost no physical impairment. Although it is used to shrug the shoulder, there are other muscles that also perform the same task. The muscle is also easy to free from its natural position and to move near the heart. Finally, it has a single main blood supply and a single nerve, which, from a technical standpoint, makes it easy to work with.
As discussed earlier, the major problem of muscle fatigue can be overcome, at last partially, by a process of conditioning the muscle to be used with electrical stimulation. Recent work also showed that a delay period after the muscle to be used is formed into its new shape is very important. Prior workers in the field had begun to use the reformed muscle immediately without realizing that such immediate use dramatically affected the ability of the muscle to receive a blood supply, and therefore resist fatigue, because the forming step ligated much of the collateral blood sources. It was later found that a period of delay allowed the formed muscle to recover somewhat from the trauma of the forming operation and to develop new channels of collateral circulation.
Other problems arose in the prior art when a skeletal muscle was used to actually replace part of the heart or the aorta. One suggestion to overcome these problems was to form a hydraulic pouch with the skeletal muscle pedicle and use that pouch to power an assist device rather than to directly connect the muscle to the heart or the aorta. The potential for hydraulic pouches is reported, for example, in Mannion, J. D., et al, "Hydraulic Pouches of Canine Latissimus Dorsi--Potential for Left Ventricular Assistance", 91(4) J. THORACIC & CARDIOVASC. SURG. 534 (April, 1986), incorporated herein by reference. That paper discloses the construction of latissimus dorsi muscle pouches in the form of a multi-layered conical spiral. The experiment there was designed to determine if any combination of electrical conditioning, vascular delay, and a multi-layered pouch would minimize muscle fatigue and permit an auxiliary ventricle to assume a portion of left ventricular function.
Another group reported on using skeletal muscle as a cardiac assisting device in conjunction with an extra-aortic balloon. The muscle there was formed as a pedicle around a bladder that was connected to a T-shaped system. The T-part of the system surrounded a dacron conduit that had been connected to the aortic blood flow by anastomosis at both ends. The T-part attached to the dacron so as to form a leak-proof system which was then filled with fluid. Thus, contraction of the muscle forced more fluid into the T-part which thereby squeezed the dacron conduit and assisted the aorta in pumping. See Nielson, I. R., et al, "Skeletal Muscle-Powered Cardiac Assist Using an Extra-Aortic Balloon Pump," Biochemical Cardiac Assist--Cardiomyoplasty and Muscle Powered Devices, Chiu, R. C. J., Ed., Futura Publishing, 19 (1986).
Despite all of the advances of the prior art, the solutions provided so far suffer from several problems. Either direct transection or anastomosis of the aorta is necessary (as in the previous paragraph) with all of the dangers incumbent upon such a procedure (interruption of blood flow, risk of infection, etc.) or no adequate method has been developed for using the idea of a muscle pouch in conjunction with a hydraulic pump system, or both. Further, if training of a skeletal muscle is needed, a series of operations have been required, multiplying all of the risks attending surgery.
Accordingly, there exists a need for an apparatus usable as an autologous biologic pump motor to assist in blood circulation, usable for an extended period of time, that avoids the dangers of both transection or anastomosis of the aorta and multiple operations.